330 BELL SVSTEM TECHNICAL JOURNAL 



transmitting medium, because they contain frequency components 

 which may easily extend into the range allotted to neighboring carrier 

 bands. For this reason it is customary in carrier telegraph practise 

 to make use of a transmitting filter to cut off these interfering fre- 

 quencies. The voltage impressed on this filter is substantially 

 rectangular in outline but after passing the filter it has a shape which 

 is approximately similar to curve C in Fig. 1, and which, therefore, 

 produces less interference than a half-c>cle sine wave. 



L.\ND Lines 



The case of land lines is somewhat different from the case discussed 

 previously because it is not economically desirable to utilize the full 

 frequency range av'ailable. In other words, the great expenditure 

 for terminal apparatus that may be proper in the case of submarine 

 cables and long distance radio circuits is not warranted. In land 

 circuits the highest frequencies transmitted are considerably greater 

 than the required line speed. When this is the case, it is usually 

 possible and desirable to make use of the available range to increase 

 the steepness of the received w^ave. A steep wave front results in 

 prompt operation of the receiving relay and this in turn results in 

 minimum distortion. If a half-cycle sine wave were to be employed 

 instead of the usual rectangular wave or if a network were to be 

 employed which were to round off the wave to the extent indicated 

 in I'^ig. 1, the received wave would necessarily lose a great part of its 

 steepness and as a consequence the response of the receiving relay 

 would be less positive and the signals would be distorted. It will, of 

 course, be understood that by means of suitably proportioned net- 

 works the wave can be rounded just enough to meet the interference 

 requirement, still retaining sufficient steepness to insure prompt 

 operation of the receiving relay. Therefore, rounding by means of 

 networks is preferable. 



If it should be desirable and praclical)le to utilize the frequency 

 range to its fullest, what has been said abo\c about a distortionless 

 line holds without any substantial modification and it would, in 

 that case also, be more advantageous to use a wave rounded by means 

 of suitable networks than to impress on the line a wave of the half- 

 cycle sine form. 



SuBM.\RiNE Cables 



In the case of submarine-cable telegraphy, there is a limitation on 

 voltage which has not been emphasized in the simple direct-current 

 case discussed above. The voltage which may be impressed on the 



